Tag: mako

There is great debate within the scientific community with regards to the true ancestral origins of the modern White Shark Carcharodon carcharias. There currently exist two main hypotheses on the correct phylogenetic placement of C. carcharias. Both hypotheses talk in terms of sharing a more recent common ancestor with an extinct species (Smith 1994). The older of the two hypotheses suggests that C. carcharias is descendant from the ancestral lineage of megatoothed sharks such as Carcharodon megalodon and that the megatoothed sharks should be placed within the family Lamnidae. The more recent hypothesis suggests that C. carcharias is descendant from the ancestral lineage of the extinct Mako shark Isurus hastalis and that the megatoothed sharks should be separately placed within the family Otodontidae. It is important to be clear that neither theory suggests an absolute direct descendancy from the afore-mentioned species as the scarcity of fossil evidence could not support such a claim.

Proponents of the megatoothed descendancy hypothesis (Applegate and Espinosa-Arrubarrena, 1996; Gottfried et al., 1996; Martin, 1996; Gottfried and Fordyce 2001; Purdy et al., 2001) base their conclusions upon aspects of shared tooth morphology between the modern C. carcharias and principally the extinct C. megalodon. These similarities include: similar tooth morphologies between juvenile C. megalodon and adult C. carcharias; fine tooth serration in both adult C. carcharias and C. megalodon; shared chevron-shaped neck area on the lingual surface of the upper anterior teeth, a mesially inclined large intermediate tooth and second anterior teeth symmetry (Gottfried et al., 1996; Gottfried and Fordyce, 2001; Purdy et al., 2001). Under this phylogenetic regime the megatoothed sharks are placed within the family Lamnidae and retain their genus Carcharodon.

The evolutionary history of Carcharodon megalodon is well understood from a large quantity of fossil evidence including many transitional specimens. The direct ancestry of C. megalodon can be traced at least as far back as the late Pliocene in the form of the large mackerel shark Otodus obliqus although if Cretolamna appendiculata is considered a chronospecies then this ancestry can be extended into the Lower Cretaceous. O. obliqus had large non serrated teeth with distinctive side cusps. During the middle Eocene the side cusps of O. obliqus reduced in size and the edges developed slight serration. This Eocene species is classified as Carcharocles auriculatus. A massive increase in shark body size accompanied by further reduction in teeth cusps and increased development in serrated edges marks the introduction of Carcharocles angustidens during the late Oligocene. Carcharodon megalodon finally evolved from C. angustidens during the early Miocene and is characterized by a further increase in body size, further development in serrated tooth cutting edges as well as a complete loss of teeth side cusps. The megatoothed descendancy hypothesis suggests that the origin of Carcharodon carcharias is derived in a form of dwarfism of Carcharodon megalodon (Ehret et al 2009).

The Isurus hastalis descendancy hypothesis argues that owing to a shared overall tooth shape and labio-lingual flattening in tooth morphology in both the later ‘broad-form’ I. hastalis (specifically the suggested transitional fossil Isurus xiphodon as described by Purdy et al. 2001) and Carcharodon carcharias, the megatoothed sharks should be viewed as a distinct and non related taxon (Casier 1960). This phylogenetic regime would see the megatoothed sharks placed in the separate family Otodontidae, containing other extinct genera such as Otodus and Parotodus, with Carcharodon megalodon being renamed as Carcharocles megalodon (Casier, 1960; Glickman, 1964; Capetta, 1987). Proponents of the Isurus hastalis descendancy hypothesis also argue that not only are tooth serrations in the megatoothed sharks much finer than those found in C. carcharias but that megatoothed sharks teeth lack enameloid in the neck area whereas C. carcharias does not (Nyberg et al. 2006).

The evolution of Isurus hastalis itself is reasonably well documented in the fossil record. All Mako sharks can find their ancestry in the Eocene epoch (approximately 50mya) with the arrival of Isurus praecursor. During the Oligocene epoch a new Mako shark, Isurus desori, appears in the fossil record. Fossils of Isurus desori are found to be almost cosmopolitan in their distribution and it is from this species that Isurus hastalis is likely to have evolved during the early Miocene. Initial forms of Isurus hastalis are relatively small and considerably longer than their width, thus these initial forms are often referred to as ‘narrow-form’ with later examples being referred to as ‘broad-form’ as their width increases in the mid to late Miocene (Alter 2013).

Casier (1960) makes suggestion of a possible transitional fossil in Isurus escheri where teeth were found to show slight fine marginal serration. Isurus escheri inhabited the waters of the Atlantic Ocean around the mid Miocene (approximately 10mya) and likely derived from the ‘narrow-form’ Isurus hastalis. Unfortunately it would appear that Isurus escheri would be an evolutionary dead-end as fossil evidence of their existence disappears within just a few million years.

True Carcharodon carcharias fossils with all modern characteristics represented have been dated back to the late Miocene with specimens being recovered from California, Maryland and Japan showing that by this time C. carcharias was already thriving across the Pacific and Atlantic Oceans (Gottfried and Fordyce, 2001; Stewart, 1999, 2000, 2002; Hatai et al., 1974; Tanaka and Mori, 1996; Yabe, 2000). A fossil recovered from Peru and described by Muizon and DeVries (1985) was suggested as another transitional fossil in favour of the Isurus hastalis descendancy hypothesis on account of weak tooth serrations but this evidence is countered by Purdy (1996) and Purdy et al. (2001) who observe that this fossil (known internationally as the Sacaco sp. on account of its discovery in the Pisco formation of the Sacaco basin, Peru) is predated by the aforementioned C. carcharias discoveries.

Based upon the simplified evidence presented it would seem that the decision is of only two possible phylogenies. It seems this view is now over-simplified as new fossil and genetic evidence (Martin 1996; Martin et al 2002) shows that the Carcharodon lineage may have split from that of the Mako sharks much earlier than suggested here and that Isurus was not in fact a true Mako but truly of the Carcharodon lineage and should therefore be placed in the genus Cosmopolitodus to indicate as such (Glikman 1964).

For now there remains great scope for research into the ancestry of Carcharodon carcharias and it is likely that debate over the matter will continue for decades to come.

In addition to the literature cited below I would refer the reader to an excellent online article by Steven A. Alter based on his many years as a collector/dealer:

Gottfried, M. D., and R. E. Fordyce. 2001. An associated specimen of Carcharodon angustidens (Chondrichthyes, Lamnidae) from the late Oligocene of New Zealand, with comments on Carcharodon interrelationships. Journal of Vertebrate Paleontology 21:730–739.

Stewart, J. D. 1999. Correlation of stratigraphic position with Isurus-Carcharodon tooth serration size in the Capistrano Formation, and its implications for the ancestry of Carcharodon carcharias. Journal of Vertebrate Paleontology 19(3, Supplement):78A.

ABOUT:

Daniel is a marine biologist with a passion for adventure. As a professional scientist he has worked in many remote locations around the world from isolated tropical volcanic islands to sub-arctic shorelines. When not pursuing his scientific interests he can usually be found seeking adventure in the mountains or on the ocean waves.